Conventional image transfer materials for thermal recording have base films made of, for example, a polyester, polypropylene, cellophane, acetate, polycarbonate or paper, and above all, polyester films. Polyethylene terephthalate (PET) films have been especially widely used.
In recent years, due to the need for a more highly precise printing quality as can be seen in the means for achieving a higher energy transfer efficiency from a thermal printing head, and also due to the increase in the thickness of the coloring material layer to suit multiple utilization, the base is required to be thinner. However, the above mentioned PET base is insufficient in heat resistance and rigidity, and is deformed under tension. Thus, a high modulus base is demanded. From this point of view, the use of a film made of aromatic polyamide having high rigidity is proposed in JP-A-60-174694, JP-A-61-237687 and JP-A-63--107588.
JP-A-04-318034 states that, for an image transfer material for thermal recording using an aromatic polyamide film, if the thermal shrinkage at 200.degree. C. is maintained at 5% or less, the wrinkling caused by film shrinkage can be desirably controlled.
In addition, from the standpoint of environmental protection, in recent years, the movement to allow multiple utilization has also become dominant in the area of image transfer materials for thermal recording. As a multiple utilization technique, a layer of accumulated coloring material is formed inside a coloring material layer or a plurality of coloring material layers are formed, so as to limit the release of coloring material during each of multiple printing stages. In any case, since the amount of the coloring material mounted on the base per surface area of the base must be increased, the base must be thinner to keep the total thickness at less than a certain value.
A recording method which can be employed utilizes a differential running speed mode in which a color receiving sheet and an image transfer material are caused to travel at different respective speeds in order to save the image transfer material used. In this case, since the traveling speed of the image transfer material is low and it is heated longer, the base is required to be higher in heat resistance than hitherto. Furthermore, the difference in traveling speed between the color receiving sheet and the image transfer material causes large friction, and even a slight dimensional change of the image transfer material causes folding and wrinkling, adversely affecting the printing characteristics.
However, due to the modulus of PET, there is a limit as to how thin a PET film can be made. Moreover, it is necessary to have an adhesion preventing layer formed on the head side so as to prevent adhesion; otherwise the film is thermally softened and stuck to the thermal printing head when thermal energy is given from such a head. Furthermore, PET is insufficient in heat resistance and not suitable for the higher printing quality described later.
To meet the need for a higher printing speed, the head must be enhanced in energy density, and as a result, the image transfer material tends to be higher in temperature. Major types of recording methods include the melt transfer type and the dye sublimation type, and it is said that the latter is more advantageous for achieving a higher printing quality by higher precision. However, a dye sublimation type recording method generally requires high energy. Furthermore, while halftone expression is one of the largest factors for requiring higher precision, such techniques as concentrated dot heating method and graded release of coloring material express images by changing the quantity of given energy, and so a material capable of withstanding high energy, i.e., with high heat resistance is required. An aromatic polyamide film is advantageous in having a high modulus and high heat resistance. However, the demand for further higher printing quality increases, and to meet the demand, the image transfer material must be enhanced in rigidity and minimized in thermal dimensional change as far as possible, because it must be thinner for more accurate and efficient energy transfer from the head, and because the productivity in processing into the image transfer material should not be sacrificed.
The increasing practice of color printing and multi-head printing and the adoption of the line head tend to demand wider image transfer materials. Thermal dimensional change, especially its irregularity causes curving and wrinkling, to impair the printing accuracy and printing quality, and this has a remarkably adverse effect in wider image transfer materials.
In relation to the problem of thermal dimensional change, JP-A-04-318034 mentioned above simply states that the thermal shrinkage should be preferably kept at 5% or less, and contains no disclosure as to how to specifically realize this, nor does it present any discussion or give technical data concerning thermal dimensional change. Moreover, JP-A-61-237687, also mentioned above, describes a case in which an image transfer material limited in thermal shrinkage is obtained by using an aromatic polyamide with a special structure, but does not refer to the in-plane irregularity of thermal dimensional change caused by a limit to the control applied in the production of aromatic polyamide film (solution casting), and as can be seen from the respective examples described in the patent gazette, such a special structure is insufficient in Young's modulus, necessitating the use of a thick film only, and also results in poor printing characteristics.